New water heater

Our water heater is busted. The plumber said we can keep using it as is, but it will continue to leak. Also, our vent isn't up to code (not surprising). So it's going to cost $170 to get it replaced. I was expecting $55. It costs more so that he can fix the vent, and the home warranty people don't pay for him to take out the old water heater. That doesn't make sense to me. Of course the old one will need to be removed. Why don't they pay for that? So if we add in the cost of the year's home warranty, it cost us about $500 for a new water heater. A little more than if we had just bought it on our own, but we renewed the home warranty because of the furnace.
I am starting to debate whether or not we need to renew it again. It comes up for renewal in August. That's too early for the furnace to be needed, but since it is almost 20 years old, I feel really uncomfortable just trusting that it will start. It was starting to act a little funny toward the end of last season. I know we can't afford the $3-5k (according to one website) for a new furnace should this one blow out next winter. But do we want to spend the $400 or so on a chance that we will need it? So far, we have used our home warranty exactly twice. Once when the stove stopped working and now with the heater. So once a year. Doesn't really seem worth it, but the furnace honestly frightens me. So I dont know. I guess we will have to debate it.